In this study, we plan to test a novel drug to prevent aGvHD. This drug, known as abatacept, specifically blocks the activation pathway critical to T cell function known as "T cell costimulation." In particular, it blocks the CD28-mediated costimulation pathway that is critical for optimal T cell activation and proliferation. My research group has done extensive pre-clinical work with this compound. Our work has demonstrated its efficacy in inducing immune tolerance after transplantation in both mouse models and primate models. In addition, patient trials have demonstrated that blocking CD28-directed T cell costimulation can prevent T cell-mediated diseases, including rheumatoid arthritis and psoriasis, and can improve solid organ transplant acceptance. Abatacept is currently FDA approved for use in rheumatoid arthritis. Given this drug's safety and efficacy profile, we have been granted an IND-exemption from the FDA for the inclusion of abatacept in a GvHD-prevention strategy.